Many types of measurement instruments have been used to determine the clearance between machine elements. The most simple mechanical types are generally referred to as "feeler" gauges. A feeler gauge contains a plurality of gauged elements of known thickness that are interposed in the clearance gap to be measured. Through trial and error, the gauge element of known thickness is matched to the gap and, thus, the clearance of the gap determined. For reasons of accuracy, these materials are usually very hard, non-resilient steel which will not compress during use.
In setting the height of a machine tool above the workpiece, it is necessary to establish either a working clearance between the tool at its retracted position and the workpiece, or to establish a reference point a known distance from the workpiece in order to "zero" other measurement components within the machine.
During "setup" prior to beginning machining operations, the machine is set as described above, usually by two different methods; either the machine tool is brought down into contact with the workpiece and that point determined as zero clearance between the tool and the workpiece, or a feeler gauge is used to establish a pre-determined clearance between the tool and the workpiece by inserting the gauge material between the tool and the workpiece and bringing the tool down into contact with the gauge element. Both of these procedures, however, require bringing the tool into contact with either the workpiece or the hard non-resilient material of the feeler gauge. In using either of these procedures, a delicate cutting tool can often be damaged or damage can be done to the workpiece causing both the expense of the damaged parts and loss of time.